1. Field of the Invention
The present invention relates to structure of a clamping apparatus of a plate for a printing machine; more specifically, a clamping apparatus of a plate for a printing machine which can carry out diagonal image adjustments easily, as well as pulling a plate with adequate tension, depending on the type of the plate.
2. Description of the Prior Art
As shown in FIG. 6, a plate 10 is rolled on a plate cylinder 60 which is used for offset printing. Offset printing is carried out by supplying both ink and dampening solution to the plate 10. A clamping apparatus is used for fixing the plate 10 on the plate cylinder 60. A cut-out part 61 is formed in the plate cylinder 60, and both a leading edge side clamping base 62 and a tail edge side clamping base 63 are provided in the cut-out part 61.
A leading edge side clamp 64 is located on the leading edge side clamping base 62. A leading edge side part 10a of the plate 10 is clamped between the leading edge side clamping base 62 and the leading edge side clamp 64. In the same manner, a tail edge side clamp 65 is located on the tail edge side clamping base 63. A tail edge side part 10b of the plate 10 is clamped between the tail edge side clamping base 63 and the tail edge side clamp 65.
In a typical procedure, in order for the plate 10 to fit on a surface of the plate cylinder 60 by applying tension, both the tail edge side clamping base 63 and the tail edge side clamp 65 are moved in a direction shown by arrow 90. A conventional structure of the clamping apparatus of a plate to apply tension to the plate 10 is described as below.
Firstly, the structure of a prior art plate cylinder as disclosed in Japanese Patent Publication No. 70145 of 1992 (Hei 4-70145) is shown in FIG. 7A. As shown in FIG. 7A, the tail edge side 10b of the plate 10 is clamped between a tail edge side clamp 75 and a tail edge side clamp base 73 provided in the cut-out part 61. FIG. 7A is a view showing a condition when the plate 10 is not under tension, and as a result is not fit on a surface of the plate cylinder 60.
A plurality of coil springs 70 are provided to engage the tail edge side clamp base 73, whereby the tail edge side clamp base 73 is pushed in the direction of the arrow 90 by the coil springs 70. A direction of expansion and contraction of the coil springs 70 is perpendicular to a shaft of the plate cylinder 60, and the coil springs 70 are arranged in a direction along the shaft of the plate cylinder 60. A plurality of spring adjustment bolts 71 are attached to each of the coil springs 70. The spring force of the coil springs 70 is adjusted by rotating the spring adjustment bolts 71, each of which is threaded and passes through the tail edge side clamp base 73.
A convex part 74 is formed underneath the tail edge side clamp base 73. A circular crank shaft 76 is positioned between a step 78 formed in the cut-out part 61 of the plate cylinder 60 and the convex part 74. The circular crank shaft 76 includes a flat surface 77. In order to apply tension to the plate 10 the circular crank shaft 76 is rotated so that the flat surface 77 faces the convex part 74, with the tail edge side clamp base 73 being moved in the tension applying direction (shown by the arrow 90) by the coil springs 70.
In other words, as shown in FIG. 7B, the plate 10 is fitted on the surface of the plate cylinder 60 as a result of pulling the plate 10 by moving the tail edge side clamp base 73 to a position right beside the flat surface 77 on the circular crank shaft 76, because the tail edge side clamp base 73 is always pushed in a direction shown by the arrow 90 by a strong spring force of the coil springs 70. In this manner, the plate 10 is fitted on the plate cylinder 60.
Another structure of a prior art clamping apparatus of a plate for a printing machine is disclosed in Japanese Patent Publication No. 5165 of 1994 (Hei 6-5165). As shown in FIG. 8A, the tail edge side 10b of the plate 10 is clamped between a tail edge side clamp 85 and a tail edge side clamp base 83 provided in the cut-out part 61. FIG. 8A is a view showing a condition when the plate 10 is not under tension, and as a result is not fitted on the surface of the plate cylinder 60.
Both sides of the tail edge side clamp base 83 are supported by brackets (not shown), and a cam 86 is rotatably supported by the brackets. The tail edge side clamp base 83, the tail edge side clamp 85 and the cam 86 are formed as a unit movable in the directions shown by the arrow 90 and the arrow 91.
Additionally, push pins 82 are passed through the tail edge side clamp base 83, and one end of the push pins 82 are in contact with the cam 86. The other end of the push pins 82 are provided with flanges 84 which are engaged by springs 80. The springs 80 are positioned between the flanges 84 and supporting members 88 having a cylinder shaped profile and an "H" shaped section, and the supporting members 88 are in contact with an inner wall 66 of the cut-out part 61. Adjustment screws 81 are provided to adjust the spring force of the springs 80. Adjustment of the spring force is carried out prior to assembly of the parts, because the adjustment can not be done after assembly.
In FIG. 8A, a flat surface 87 of the cam 86 is in contact with the push pins 82. In order to apply tension to the plate 10, the cam 86 is rotated with a tool 89 (FIG. 8B). As a result of the rotation, although the push pins 82 are pushed by the cam 86, the push pins 82 are pushed in the direction shown by the arrow 91 by a spring force of the springs 80.
Therefore, the cam 86 is moved in the direction of the arrow 91 due to its rotation, so that the tail edge side clamp base 83 and the tail edge side clamp 85 are moved in the direction of the arrow 91 with the cam 86 as a unit. Thus, the plate 10 is pulled toward a direction of the arrow 91. As shown in FIG. 8B, the springs 80 are compressed by a pushing force of the cam 86, and the adjustment screws 81 are moved slightly in the direction of the arrow 90. Thus, the plate 10 is fitted on the plate cylinder 60 by applying tension to the plate 10 as described above.
However, conventional structures of the clamping apparatus described above have the following problems to be resolved. Diagonal image adjustment of the plate 10 is carried out after rolling the plate 10 onto the plate cylinder 60 by moving both the tail edge side clamping base 63 and the tail edge side clamp 65, both of which clamp the tail edge side part 10b of the plate 10 (see FIG. 6), in a direction parallel to the shaft of the plate cylinder 60.
In the prior art apparatus shown in FIG. 7A and FIG. 7B, the tail edge side clamp base 73 is pushed strongly in the direction of the arrow 90 by the coil springs 70. Therefore, it is not easy to carry out diagonal image adjustment of the plate 10 because of the difficulty of moving the tail edge side clamp base 73 in a direction parallel to the shaft of the plate cylinder, due to a contact pressure generated between the convex part 74 and the circular crank shaft 76.
Referring to the prior art apparatus shown in FIGS. 8A and 8B, not much contact pressure is generated between the cam 86 and the push pins 82 in the configuration shown in FIG. 8A. In other words, no contact pressure is generated between the cam 86 and the push pins 82 because the pushing of the push pins 82 in the direction of the arrow 91 is limited due to contact between the adjustment screws 81 and the supporting members 88. Therefore, diagonal image adjustment of the plate 10 can be easily carried out by moving the tail edge side clamp base 83 in a direction parallel to the shaft of the plate cylinder.
However, the prior art apparatus of FIGS. 8A and 8B has the following problems. Strength of the plate 10 is dependent on the type of the plate. It is preferable to apply adequate tension depending upon the type of the plate 10. In the prior art shown in FIG. 7A and FIG. 7B, the tension to the plate 10 can be adjusted by varying the spring force of the coil springs 70 through rotation of the spring adjustment bolts 71. On the contrary, in the prior art shown in FIG. 8A and FIG. 8B, the tension can not be adjusted after assembly because the adjustment screws 81 are hidden inside of the supporting members 88 and the plate cylinder 60.
Also, it is not possible to vary the tension applied to the plate 10 at various locations along the length thereof because the adjustment screws 81 can not be adjusted. In the first prior art shown in FIG. 7A and FIG. 7B, the spring adjustment bolts 71 are arranged substantially in a direction parallel to the direction of the shaft of the plate cylinder 60. It is therefore possible to apply different tensions to the plate 10 at various positions along the length thereof by adjusting the spring adjustment bolts 71. On the contrary, the prior art shown in FIG. 8A and FIG. 8B can only apply a uniform tension to the plate 10 along the axis of the shaft of the plate cylinder 60.